If f(x, y) = 2x2+ 52 and u is the unit vector in the direction of which of the following expressions equals Duf(x, y) a. 2 ( x + 21) +5(1+ 13h) -(2x2+ 5y?) lim h-0 h 2 ( x + 12n ) +5(1+ 121)"-(2x2 + 5y?) O. lim 1-0 h 2 (x + 13 , 21 ) + 5 (1 + ;1 )-- (2x2 + 54- ) C. lim h-0 h d. lim h-0 h e. None of the given choices